1. Field of the Invention
This invention relates generally to an apparatus for controlling the frequency response of an electrical signal filter and more particularly to a programmable resistor-capacitor filter suitable for VLSI integrated circuits.
2. Description of Prior Art
Electrical signal filters comprised of a combination of electrical impedances such as resistors(R) and capacitors(C) are well known and are used extensively in all types of electrical and electronics apparatus.
Typically RC filters fall into two classes, low-pass filters and high-pass filters. For example, FIG. 1 discloses a simple low-pass filter comprised of a single fixed resistor 10 and capacitor 12 connected between an input terminal 14 and an output terminal 16. The resistor 10 is connected in series between input and output terminals 14 and 16 while the capacitor is connected between a common junction 18 between the RC components and a point of reference potential shown as ground.
FIG. 2 on the other hand depicts a simple high-pass filter consisting of a resistor 10 and a capacitor 12 but now the resistor 10 is returned to ground from junction 18 while the capacitor 12 is connected in series between the input and output terminals 14 and 16. The filter circuits shown in FIGS. 1 and 2 have a single corner of frequency response depending upon the selected values of the resistor 10 and the capacitor 12.
Where there is a need to vary the corner of the frequency response low-pass and high-pass filters on demand, a plurality of resistors and capacitors have been used which are selectively switched in and out of a filter circuit. For example, FIG. 3 is illustrative of a switched resistor approach for implementing a high-pass filter and includes a single capacitor 12 and a plurality of fixed resistors 101, 102, . . . 10nxe2x88x921, and 10n which are connectable between one side of the capacitor 12, as shown by reference numeral 22 and ground by means of a set of electrically controlled switch devices 241, 242, . . . 24n. These switches are respectively closed or opened in response to control signals applied from a control source, not shown, to terminals 261, 262, . . . 26n. The control source may be, for example, a multi-bit control register where each bit determines which resistor is to be connected into the circuit. In the circuit configuration shown in FIG. 3, one or all of the fixed resistors 101 . . . 10n could be connected in parallel between 22 and ground. In such a configuration, a large coupling capacitor 28 is typically included between the output terminal 16 and the filter components to provide DC isolation due to the shunting effect of the high-pass filter resistors 101, . . . 10n.
In a like manner, a plurality of capacitors can be utilized in a switched circuit configuration for implementing a high-pass filter. This is shown in FIG. 4. Now a plurality of fixed capacitors 121, 122, . . . 12n and series connected switches 241, 242, . . . 24n are connectable in parallel between the input terminal 14 and one side of the grounded resistor 10 as shown by reference numeral 23. As before, the controlled switches 241, 242, . . . 24n are open and closed in response to control signals applied to terminals 301, 302, . . . 30n. Thus, depending upon which capacitor, for example, 121 or a combination of parallel capacitors 122, . . . 10n is switched into the circuit a high-pass filter characteristic can be selectively programmed or changed as desired. Again, a large coupling capacitor 28 is connected between the filter and output terminal 16 to provide the required isolation between the low-pass filter and the circuitry 10 connected to the output terminal 16.
Where such circuitry is desired for use in connection with on-chip digital control of RC filters fabricated in a VLSI integrated circuit, both approaches as shown in FIGS. 3 and 4 suffer from an inherent disadvantage in that relatively large coupling capacitors are required to provide the necessary DC isolation due to the shunting effect of the resistors. Also, in the switched capacitor approach, as shown in FIG. 4, a relatively large area is required on the chip due to the need of multiple capacitors which by their very nature consumes an inordinate amount of space, i.e. area, on the structure.
Accordingly, it is an object of the present invention to provide improvement in electrical signal filters.
It is another object of the invention to provide improvement in programmable RC filters which are suitable for very large scale integrated circuits.
It is yet another object of the invention to provide a programmable RC filter which overcomes both size and isolation limitations of the prior art.
These and other objects are achieved by a modified switched resistor approach which overcomes the limitations of the prior art and involves the use of a resistive type voltage divider network where one of the resistors comprises one element of the filter and wherein the voltage divider also provides a DC bias to the circuitry coupled to the output of the filter.
In one aspect of the invention, there is provided a filter comprised of a resistive component and a capacitive component connected together in an electrical filter circuit configuration between an input terminal and an output terminal and having a predetermined frequency response and wherein the resistive component comprises one element of at least one voltage divider circuit connected across a DC voltage source so to provide a substantially fixed DC voltage of a predetermined magnitude at the output terminal
In another aspect of the invention, there is provided a filter comprised of a capacitor connected between an input terminal and an output terminal, and a plurality of switched voltage dividers being connected between a source of DC voltage and point of reference potential, each of said voltage dividers including at least two series connected circuit elements having an electrical resistance chracteristic and having a common circuit connection therebetween, said common circuit connection being commonly connected to said capacitor and said output terminal and thereby providing a DC bias voltage at said output terminal.
In yet another aspect of the invention, there is provided a filter comprised of a high-pass filter section including a first capacitor connected between an input terminal and an intermediate circuit junction, and a plurality of switched voltage dividers each including at least a pair of resistive circuit elements connected across a source of DC voltage and wherein said at least one resistive element is connected to a point of reference potential or ground, each of the voltage dividers having a series circuit connection commonly connected to the intermediate circuit junction, and wherein the voltage dividers are selectively switched in and out of circuit relationship such that one resistive element of said pair of resistive circuit elements and said first capacitor form a filter circuit having a programmable high-pass frequency response, and a low-pass filter section including a plurality of switched resistive circuit elements selectively connected in parallel between the intermediate circuit junctions and an output terminal, and a second capacitor connected between one side of said switched resistive elements common to the output terminal and said point of reference potential or ground, and wherein at least one resistive element of said switched resistive elements is switched into circuit relationship with the second capacitor so as to form a filter circuit having a programmable low-pass frequency response, and wherein a DC bias voltage is provided at said output terminal.
Where active semiconductor devices such as field effect transistors are utilized as the resistive elements of the filter, such an implementation utilizes very little area and consumes only a small amount of power on a semiconductor integrated circuit chip.